fix handling of loading auth backend

When a non existing auth backend was configured, the action router ran
into an infinie loop exception. The reason was that the denied action
required a configured auth system, but denying access should always
work.

Interestingly the problem did not occur when the auth backend signalled
a failure to load. This was because the auth backend was not properly
deinitialized. This is now done.

To aid debugging similar problems, fatal errors are now logged through
the logging mechanism in the action router

inc/Action/Denied.php

@@ -11,7 +11,7 @@
  *
  * @package dokuwiki\Action
  */
-class Denied extends AbstractAclAction
+class Denied extends AbstractAction
 {
     /** @inheritdoc */
     public function minimumPermission()

inc/ActionRouter.php

@@ -156,6 +156,7 @@ protected function handleFatalException(\Exception $e) {
         if(defined('DOKU_UNITTEST')) {
             throw $e;
         }
+        ErrorHandler::logException($e);
         $msg = 'Something unforeseen has happened: ' . $e->getMessage();
         nice_die(hsc($msg));
     }

inc/auth.php

@@ -56,7 +56,7 @@ function auth_setup() {
 
     if ($auth->success == false) {
         // degrade to unauthenticated user
-        unset($auth);
+        $auth = null;
         auth_logoff();
         msg($lang['authtempfail'], -1);
         return false;